Slide valve for high-viscosity pumps

ABSTRACT

A slide valve for dense or viscous materials, e.g. concrete, has a slide element connected to a piston rod which extends out of the valve housing through a seal and carries a piston received in a cylinder which is clamped to the housing around the seal by a clamping nut. The cylinder has an outer actuation chamber while an inner annular chamber between the piston and the seal is a flushing compartment connected to a reservoir of liquid through a single duct. As the piston moves toward the seal, the liquid in the flushing compartment is expelled, taking with it any foreign matter which has gotten past the seal, and as the piston movement is reversed this compartment is filled with the liquid via this duct to flush the rod again. The valve element, valve guides, end stops, and sleeves surrounding the piston rods are all easily removable and replaceable to facilitate servicing of the valve.

United States Patent Verheul [54] SLIDE VALVE FOR HIGH-VISCOSITY PUMPS[72] Inventor: Cornelis Verheul, Amsterdam, Netherlands [73] Assignee:Friedrich W. Schwing, Wanne-Eickel,

Germany [22] Filed: Feb. 3, 1970 [21] Appl. No.: 8,381

[30] Foreign Application Priority Data Feb. 5, 1969 Germany ..P 19 05706.8

[52] US. Cl. ..137/238, 92/5 R, 92/86, 92/87, 137/607, 251/62, 251/326,277/2 [51] Int. Cl ..F16k 41/00, F16k 3/314 [58] Field of Search ..92/5,86, 87; 251/636, 326; 277/2; 184/18; 137/237, 238, 240, 312

[56] References Cited UNITED STATES PATENTS 1,317,563 9/1919 Eckenroth..277/110 X 2,478,811 8/1949 Downs ..251/327 X 2,628,872 2/1953 Moore..92/153 X Feb. 8, 1972 Primary Examiner -M. Cary Nelson AssistantExaminer-Richard Gerard Attorney-Karl F. Ross [5 7] ABSTRACT A slidevalve for dense or viscous materials, e.g. concrete, has a slide elementconnected to a piston rod which extends out of the valve housing througha seal and carries a piston received in a cylinder which is clamped tothe housing around the seal by a clamping nut. The cylinder has an outeractuation chamber while an inner annular chamber between the piston andthe seal is a flushing compartment connected to a reservoir of liquidthrough a single duct. As the piston moves toward the seal, the liquidin the flushing compartment is expelled, taking with it any foreignmatter which has gotten past the seal, and as the piston movement isreversed this compartment is filled with the liquid via this duct toflush the rod again. The valve element, valve guides, end stops, andsleeves surrounding the piston rods are all easily removable andreplaceable to facilitate servicing of the valve.

13 Claims, 10 Drawing Figures SHEETlUFS INVENTOIR ATTORNEY PATENIEU FEB8 I972 SHEET 2 [IF 5 FIG. 2A

FIG. 2B

ATTORNEY PATENIEI] FEB 8 B72 sum 5 0F 5 Ill III INVENTOR co E4/6L/5ATTORNEY SLIDE VALVE FOR HIGH-VISCOSITY PUMPS FIELD OF THE INVENTION Thepresent invention relates to a slide valve. More particularly, thisinvention relates to a slide valve assembly for controlling highlyviscous matter, such as concrete.

BACKGROUND OF THE INVENTION Valves of the above-mentioned type, for usewith viscous or dense materials such as concrete, include a slide valvewhose slide element is connected to a rod which passes through a seal inthe wall of the conduit or valve housing. The slide or valveclosureelement is usually mounted on two such rods which are connected togetherand actuated by a single hydraulic cylinder mounted outside the conduit.When such a valve is used in conjunction with a concrete pump, forinstance, granular, highly corrosive matter tends to work its way out ofthe conduit through the seal. To prevent this matter from contaminatingthe hydraulic cylinder and from drying and then wearing out the seal asthe drive rod reciprocates, a small stream of washing liquid, usuallywater, is directed at the rods where they emerge from the conduit.

While flushing with water carries away much of the contaminatingmaterial, the use of the technique on the case of concrete when thewater merely falls to the ground, creates a problem since the concretecan accumulate and harden, even under water, and permanently foul thearea. Moreover, a reverse leakage of the flushing liquid often occurs todilute or contaminate the viscous material being controlled by thevalve.

Another disadvantage of such a valve structure is that the exposed,usually constantly reciprocating, drive rods create an injury hazard,the seals are often ineffective and require repeated adjustment since itcan not be ascertained precisely when the seals become inoperative, andthe guide means and actuating system for the slide is inordinatelycomplex.

OBJECTS OF THE INVENTION It is, therefore, an object of the presentinvention to provide an improved slide valve of the above-mentionedtype, especially for use with high viscosity and dense materials withabrasive characteristics.

Another object of the present invention is to provide such a valve whichwears little, is easy to service, presents virtually no danger of injuryto operating personnel, and is relatively inexpensive to manufacture andmaintain.

It is also an important object of the invention to provide an improvedreliable concrete valve for use in concrete-pump installations.

SUMMARY OF THE INVENTION The above objects are attained, in accordancewith the present invention, with a valve wherein the or each control rodis provided at an end with a cylinder arrangement with a piston mountedon the rod and defining in the cylinder itself an actuating chamberremote from the slide and connected to a source of valve-operating fluidpressure, and a flushing chamber between the piston and the seal andconnected via a single duct or tube to a reservoir of flushing fluidsuch that, on displacement of the piston toward the seal, fluid isexpelled from the flushing chamber into the reservoir, and ondisplacement in the opposite direction, fresh flushing fluid e.g., alubricant alone or together with water or other liquid is drawn intothis chamber from the reservoir to rinse the rod. The tube thus servesas an intake upon displacement of the piston away from the seal and asan outlet upon displacement of the piston in the reverse direction.

The duct, according to another feature of the invention, opens below theliquid level into a relatively large, transparent reservoir, the entireflushing network being fully closed. In this manner, the amount ofmaterial washed off the rod can be constantly monitored so that thepacking nuts can be tightened when an overly large quantity of materialgets through the packings. Furthermore, the large reservoir allows thematerial washed off to settle out of the flushing liquid, or to rise toits surface if it is less dense. The reservoir may have a window throughwhich the condition of the liquid (i.e., its turbidity) may bemonitored, so that, in any event, the reservoir is at least partlytransparent.

This cylinder is in the form ofa blind tube whose closed end forms theactuating chamber and whose open end is held by the packing nut to thevalve housing or conduit. A thrust ring is placed between the open endof the tube and the outer end of the seal with its inner end bearingagainst the conduit, so that turning of the nut draws the cylindertoward the conduit and tightens the seal.

Preferably the mouth of the cylinder is formed with an outwardlyextending flange behind which the inwardly extending flange of thepacking or coupling nut engages. The mouth of the cylinder is alsoprovided with a recess defining an inwardly extending shoulder which isclamped under axial pressure against a rigid compression sleeve orbushing. The latter fits snugly but slidably within a coaxial boss uponthe valve housing, the packing or coupling nut being threaded onto thisboss. Within the boss, a yieldable gland or packing ring is compressedby the sleeve and, deformed radially inwardly and outwardly against therod and the inner wall of the boss as well as axially against the valvehousing.

The valve member, its guides, and its end stops are all replaceable andremovably mounted on the control rods, which are themselves protected byremovable and replaceable tubular shields. This construction is verylight and allows the simple and inexpensive replacement of all parts ofthe valve which are highly likely to wear, while those which would bedifficult and expensive to replace, i.e., the control rods, arecompletely protected.

In accordance with another feature of the invention, only a singlecontrol rod is provided on each valve element, and two such elements canbe mounted on a single such rod with a cylinder arrangement at each end.

The invention also provides that the rod, as this term is usedhereinabove, may comprise a coaxial arrangement of an outer wear tube orsleeve, which closely surrounds the inner rod member, extending from thepiston to the slider and completely but removably shielding the innerrod. The wear tube or sleeve represents the portion of the rodcooperating with the seal and flushed in the compartment ahead of thepiston. The wear tube or sleeve is clamped axially against the slide bythe piston-securing nuts.

DESCRIPTION OF THE DRAWING The above and other objects, features, andadvantages will become apparent from the following description,reference being made to the drawing, in which:

FIG. 1 is a partly sectional view through a valve and conduitarrangement according to the present invention;

FIG. 2A is a partly sectional view showing the valve element accordingto the present invention;

FIG. 2B is a section along line IIB-IIB of FIG. 2A;

FIG. 3A is a view of one of the valve guides, according to the presentinvention, to the same scale as FIGS. 2A and 28;

FIG. 3B is a section along line IIIB-IIIB of FIG. 3A;

FIG. 4A is an end view, to the same scale as FIGS. 2A3B, of part of thevalve seat according to the present invention;

FIG. 4B is a section along line lVB-IVB of FIG. 4A;

FIG. SA is a view similar to FIG. 1A showing an alternative embodimentof the present invention;

FIG. 5B is a section along line VB-VB of FIG. 5A; and

FIG. 6 is a partly sectional view showing an installation using thevalveof FIGS. 5A and 5B.

SPECIFIC DESCRIPTION A concrete-pump control valve 1 has a slider 2which can block one of two conduits 3. In FIG. 1 the right-hand conduitis blocked. The slider 2, as shown in FIGS. 2A and 2B comprises a flatcenter portion 2a which is symmetrical about an axis A and has twosemicircular ends 2b. Each end 2b has a radius of curvature r equal tothat of the conduits 3. To each side of the center portion 2a is weldeda tube 19 having an internal diameter d and an external diameter d Theslider 2 is carried on a pair of steel piston rods 4 which have an outerdiameter equal to d so that they just fit in the tubes 19. At each endof each of the rods 4 is a small threaded portion 4a on which a nut 4bholds a plurality of washers 40 which form a piston 12. Each rod 4passes out through the wall 18 of the conduit or through the valvehousing and through a packing 5, and thence into a hydraulic actuatingcylinder 6.

The cylinder 6 comprises a blind cylindrical tube 13 having a regularcross-sectional area and an inside diameter D larger than d which is inturn larger than 11,. A pressurizable and depressurizable chamber 13a isthus formed to the right of the piston 12 as seen in FIG. 1. By the sametoken, closed annular chamber 7 is formed to the left of this piston 12,for reasons which will be described below.

Adjacent the conduit or valve housing 18 the cylinder 13 abuts with itsend 14 a metal ring 11 that serves to compress the packing ring 5. Tothis end the cylinder 13 is formed adjacent the housing 18 with anoutwardly extending flange 15 which can be engaged by a packing nut 16that screws onto a nipple 17 formed on the housing 18.

The entire rod 4 is sheathed between both of its pistons 12 by the tube19 of the element 2 and two tubes or sleeves 10 having the same insideand outside diameters as the tubes 19. In this manner the rod 4 itselfat no point is directly exposed to wear.

The element 2 slide between a pair of guide plates 20 shown in FIGS. 3Aand 33. Each plate 20 has a relatively thin center portion 20a havingtwo oppositely directed circular cutouts 22 of radius of curvature rwhich is the same radius of curvature as the conduit 3 and the ends 2bof the elements 2. To each side of the center section 20a is a ridge 21adefining a semicylindrical recess 21 of diameter d Since the element 2is adapted to move back and forth to block and unblock one of twoconduits 3, it is clear how it will slide between these two guides 20with the semicylindrical grooves receiving the tubes 19 and 10.

Defining the limit of travel of the element 2 at each end is a pair ofstop elements 23, one of which is shown in FIGS. 4A and 48. Each element23 is in the form of a block with a throughgoing bore 21a of diameter 11and having a corner rounded at 220 with a radius of curvature equal tor.

FIGS. 5A and 5B show a valve 1 essentially the same as that of FIGS. 148except that here the valve element 2 is connected to only one rod 4 anda one-piece end stop 23 is provided. FIG. 5B shows how the plates 20 hugthe element 2.

In FIG. 6 there is shown a typical installation wherein two conduits 26both lead to a supply of concrete. Two inlet conduits 27 lead upwardlyfrom the conduits 26 to opposite chambers of a pump 27, preferably arocker-arm pump. Furthermore, both conduits 27 lead down to an outletbase 28 leading to wherever the concrete is to be pumped. Two elements2' are mounted on a common piston rod 4 extending between twopiston-and-cylinder arrangements 6 so that at one time only one conduit26 and the opposite conduit 27 can be open. Thus, as the pump 27 draws acharge of concrete up through the right-hand conduit 26 itsimultaneously expels a previously obtained charge down through theleft-hand conduit 27. Then the valve is reversed by synchronized control25 so that a charge can be drawn up through the left-hand conduit 26 andthe previously picked up charge can be pushed out through the right-handconduit 27. In this manner a relatively steady flow is created in theconduit 28, as long as the switching time for the elements 2' isreasonably short.

The valve of FIG. 1 is operated by a pump 25 which can be connectedthrough a selector valve 25 and conduits 24 to the cylinders 13 todisplace the pistons 12 therein. In normal use this valve 25 is movedback and forth to move the element 2 back and forth, as is describedimmediately above.

The chambers 7 are fully sealed except where a duct 8 which has one endin a reservoir 9 filled with shale oil or an oil-water emulsion, or evencommon oil, or other liquid which does not attack and is not attacked byconcrete so that as the piston 12 moves back and forth this flushingfluid is sucked into and expelled from the chamber 7. The reservoir 9and the tubes 8 are preferably made of transparent material so that anyforeign matter entering the chamber 7 through the seal 5 can quickly beseen. The fluid must have a density different from that of the particleslikely to get past the seal 5 so that these particles will either settleout or rise to the surface in the reservoir 9.

Should such particles become visible in either the reservoir 9 or tubes8, one or all of the packing nuts 16 is rotated to tighten the seal 5.In prior art systems it has always been necessary to simply periodicallytighten all of the seals to prevent leaks, thereby greatly increasingthe power loss caused by friction and the amount of wear of these seals.

Furthermore, should the valve 1 need overhauling, all ofthe partsexposed to wear, which are all of steel, can be quickly and simplyreplaced. The valve housing 18 must be opened at one side and all fourcylinders 13 removed. Then then the nuts 4b at one side of the valve areremoved, and the rods 4 pulled entirely out through the other side ofthe valve 1. New blocks 23, guides 20, protecting sleeves 10, valveelement 2, and seals 5 can be quickly mounted, and the devicereassembled. Such an overhaul can be carried out by a semiskilled workerin a very short time, and the necessary new parts are of nominal cost.

Iclaim:

l. A slide valve for a conduit, comprising:

a slidable valve element in said conduit displaceable into and out of aposition blocking same;

a rod connected to said element and passing out through a wall of saidconduit;

a piston on said rod external of said conduit;

a seal in the wall of said conduit tightly surrounding said rod;

a fluid cylinder external of said conduit surrounding said rod andreceiving said piston, said piston subdividing said cylinder into aflushing compartment between said piston and said seal and an actuatingcompartment on the other side of said piston;

a reservoir containing a flushing fluid; and

duct means connecting said reservoir to said flushing compartment fordrawing said fluid into said flushing compartment upon displacement ofsaid element in one direction and for expelling said fluid from saidflushing compartment on displacement of said element in the otherdirection, said cylinder being of regular cross-sectional area, thecross-sectional area of said piston being greater than that of said rod,said flushing compartment being annular and surrounding said rod, saidrod comprising an inner rod member and an outer replaceable andremovable sleeve member, said valve element being formed with at leastone tube portion having a throughgoing bore fitting said inner rodmember, said valve having one such cylinder at each end of said rod,said sleeve members extending fully between said tube portion and saidpistons and being clamped in place by same.

2. The valve defined in claim 1 wherein said cylinder includes a blindtube having a closed end forming said actuating compartment and an openend directed toward said seal.

3. The valve defined in claim 2, further comprising a clamp nutthreadedly engaging said conduit around said seal and having an internalflange, said conduit having an external flange entrained by said nut inthe direction of said conduit.

4. The valve defined in claim 3, further comprising a thrust ringbetween said open end ofsaid tube and said seal, whereby rotation ofsaid nut compresses and tightens said seal.

5. The valve defined in claim 1, further comprising a pair ofreplaceable and removable guide members flanking and slidably receivingsaid valve element.

6. The valve defined in claim 1 wherein said reservoir is at leastpartially transparent.

7. A slide valve for a conduit comprising:

a slidable valve element in said conduit displaceable into and out of aposition blocking same;

a rod connected to said element and passing out through a wall of saidconduit;

a piston on said rod external of said conduit;

a seal in the wall of said conduit tightly surrounding said rod; 7

a fluid cylinder external of said conduit surrounding said rod andreceiving said piston, said piston subdividing said cylinder into aflushing compartment between said piston and said seal and an actuatingcompartment on the other side of said piston;

a reservoir containing a flushing fluid;

duct means connecting said reservoir to said flushing compartment fordrawing said fluid into said flushing compartment upon displacement ofsaid element in one direction and for expelling said fluid from saidflushing compartment on displacement of said element in the otherdirection; said cylinder being of regular cross-sectional area, thecross-sectional area of said piston being greater than that of said rod,said flushing compartment being annular and surrounding said rod;

a pair of replaceable and removable guide members flanking and slidablyreceiving said valve element; and

at least two end-stop members engageable with said element at the limitof travel thereof, said guide members each being formed with matchingsubstantially semicylindrical grooves and said stop members being formedwith throughgoing holes, said grooves and said holes forming athroughgoing bore of regular diameter snugly slidably receiving saidrod.

8. The valve defined in claim 7 wherein said rod comprises an inner rodmember and an outer replaceable and removable sleeve member.

9. The valve defined in claim 8 wherein said valve element is formedwith at least one tube portion having a throughgoing bore fitting saidinner rod member.

10. The valve defined in claim 7 wherein said valve element is connectedto a pair of such rods.

11. A slide valve for a conduit comprising:

a slidable valve element in said conduit displaceable into and out of aposition blocking same;

a rod connected to said element and passing out through a wall of saidconduit;

a piston on said rod external of said conduit;

a seal in the wall of said conduit tightly surrounding said rod;

a fluid cylinder external of said conduit surrounding said rod andreceiving said piston, said piston subdividing said cylinder into aflushing compartment between said piston and said seal and an actuatingcompartment on the other side ofsaid piston;

a reservoir containing a flushing fluid;

duct means connecting said reservoir to said flushing compartment fordrawing said fluid into said flushing compartment upon displacement ofsaid element in one direction and for expelling said fluid from saidflushing compartment on displacement of said element in the otherdirection;

a pair of substantially flat replaceable and removable guide membersflanking and slidably receiving said element, said guide members beingformed with opposing substantially semicylindrical grooves;

a pair of end-stop members engageable with said element at the extremelimit of travel thereof, said stop members being formed withthroughgoing holes, said holes and said grooves defining an axiallyextending straight bore snugly fitting said rod, said rod having onesuch cylinder at each end and including an inner rod member carrying ateach of its ends one such piston and a sleeve member extending betweeneach of said pistons and said element, said element being formed withtube portion clamped between said sleeve members and having a borereceiving said rod member;

a nut having an internal flange and threadedly engaging said conduitaround said seal, said cylinder being a blind tube having a closed endforming said actuating compartment and an open end directed toward saidseal and formed with an external flange and an internal shoulder, saidinternal flange bearing on said external flange; and

a thrust sleeve between said internal shoulder and said seal wherebyrotation of said nut tightens said seal, said reservoir being at leastpartially transparent, said guide and end-stop members forming asubstantially circular opening corresponding to the internal diameter ofsaid conduit and in line therewith, said flushing compartment beingannular and surrounding said rod between said seal and said piston.

12. A slide valve for a conduit comprising:

a slidable valve element in said conduit displaceable into and out of aposition blocking same;

a rod connected to said element and passing out through a wall of saidconduit;

a piston on said rod external of said conduit;

21 seal in the wall of said conduit tightly surrounding said rod andprovided with means for tightening the seal upon wear thereof;

a fluid cylinder external of said conduit surrounding said rod andreceiving said piston, said piston subdividing said cylinder into aflushing compartment between said piston and said seal and an actuatingcompartment on the other side of said piston;

a reservoir containing a flushing fluid; and

a flushing duct connecting said reservoir to said flushing compartmentfor drawing said fluid into said flushing compartment upon displacementof said element in one direction and for expelling said fluid from saidflushing compartment on displacement of said element in the otherdirection.

13. The slide valve defined in claim 12 wherein said valve element isprovided with a respective rod on opposite sides thereof and extendingin the direction of displacement of said valve element, each of saidrods being provided with a respective piston, seal, fluid cylinder andflushing duct is defined.

1. A slide valve for a conduit, comprising: a slidable valve element insaid conduit displaceable into and out of a position blocking same; arod connected to said element and passing out through a wall of saidconduit; a piston on said rod external of said conduit; a seal in thewall of said conduit tightly surrounding said rod; a fluid cylinderexterNal of said conduit surrounding said rod and receiving said piston,said piston subdividing said cylinder into a flushing compartmentbetween said piston and said seal and an actuating compartment on theother side of said piston; a reservoir containing a flushing fluid; andduct means connecting said reservoir to said flushing compartment fordrawing said fluid into said flushing compartment upon displacement ofsaid element in one direction and for expelling said fluid from saidflushing compartment on displacement of said element in the otherdirection, said cylinder being of regular cross-sectional area, thecrosssectional area of said piston being greater than that of said rod,said flushing compartment being annular and surrounding said rod, saidrod comprising an inner rod member and an outer replaceable andremovable sleeve member, said valve element being formed with at leastone tube portion having a throughgoing bore fitting said inner rodmember, said valve having one such cylinder at each end of said rod,said sleeve members extending fully between said tube portion and saidpistons and being clamped in place by same.
 2. The valve defined inclaim 1 wherein said cylinder includes a blind tube having a closed endforming said actuating compartment and an open end directed toward saidseal.
 3. The valve defined in claim 2, further comprising a clamp nutthreadedly engaging said conduit around said seal and having an internalflange, said conduit having an external flange entrained by said nut inthe direction of said conduit.
 4. The valve defined in claim 3, furthercomprising a thrust ring between said open end of said tube and saidseal, whereby rotation of said nut compresses and tightens said seal. 5.The valve defined in claim 1, further comprising a pair of replaceableand removable guide members flanking and slidably receiving said valveelement.
 6. The valve defined in claim 1 wherein said reservoir is atleast partially transparent.
 7. A slide valve for a conduit comprising:a slidable valve element in said conduit displaceable into and out of aposition blocking same; a rod connected to said element and passing outthrough a wall of said conduit; a piston on said rod external of saidconduit; a seal in the wall of said conduit tightly surrounding saidrod; a fluid cylinder external of said conduit surrounding said rod andreceiving said piston, said piston subdividing said cylinder into aflushing compartment between said piston and said seal and an actuatingcompartment on the other side of said piston; a reservoir containing aflushing fluid; duct means connecting said reservoir to said flushingcompartment for drawing said fluid into said flushing compartment upondisplacement of said element in one direction and for expelling saidfluid from said flushing compartment on displacement of said element inthe other direction; said cylinder being of regular cross-sectionalarea, the cross-sectional area of said piston being greater than that ofsaid rod, said flushing compartment being annular and surrounding saidrod; a pair of replaceable and removable guide members flanking andslidably receiving said valve element; and at least two end-stop membersengageable with said element at the limit of travel thereof, said guidemembers each being formed with matching substantially semicylindricalgrooves and said stop members being formed with throughgoing holes, saidgrooves and said holes forming a throughgoing bore of regular diametersnugly slidably receiving said rod.
 8. The valve defined in claim 7wherein said rod comprises an inner rod member and an outer replaceableand removable sleeve member.
 9. The valve defined in claim 8 whereinsaid valve element is formed with at least one tube portion having athroughgoing bore fitting said inner rod member.
 10. The valve definedin claim 7 wherein said valve element is connected to a pair of suchrodS.
 11. A slide valve for a conduit comprising: a slidable valveelement in said conduit displaceable into and out of a position blockingsame; a rod connected to said element and passing out through a wall ofsaid conduit; a piston on said rod external of said conduit; a seal inthe wall of said conduit tightly surrounding said rod; a fluid cylinderexternal of said conduit surrounding said rod and receiving said piston,said piston subdividing said cylinder into a flushing compartmentbetween said piston and said seal and an actuating compartment on theother side of said piston; a reservoir containing a flushing fluid; ductmeans connecting said reservoir to said flushing compartment for drawingsaid fluid into said flushing compartment upon displacement of saidelement in one direction and for expelling said fluid from said flushingcompartment on displacement of said element in the other direction; apair of substantially flat replaceable and removable guide membersflanking and slidably receiving said element, said guide members beingformed with opposing substantially semicylindrical grooves; a pair ofend-stop members engageable with said element at the extreme limit oftravel thereof, said stop members being formed with throughgoing holes,said holes and said grooves defining an axially extending straight boresnugly fitting said rod, said rod having one such cylinder at each endand including an inner rod member carrying at each of its ends one suchpiston and a sleeve member extending between each of said pistons andsaid element, said element being formed with tube portion clampedbetween said sleeve members and having a bore receiving said rod member;a nut having an internal flange and threadedly engaging said conduitaround said seal, said cylinder being a blind tube having a closed endforming said actuating compartment and an open end directed toward saidseal and formed with an external flange and an internal shoulder, saidinternal flange bearing on said external flange; and a thrust sleevebetween said internal shoulder and said seal whereby rotation of saidnut tightens said seal, said reservoir being at least partiallytransparent, said guide and end-stop members forming a substantiallycircular opening corresponding to the internal diameter of said conduitand in line therewith, said flushing compartment being annular andsurrounding said rod between said seal and said piston.
 12. A slidevalve for a conduit comprising: a slidable valve element in said conduitdisplaceable into and out of a position blocking same; a rod connectedto said element and passing out through a wall of said conduit; a pistonon said rod external of said conduit; a seal in the wall of said conduittightly surrounding said rod and provided with means for tightening theseal upon wear thereof; a fluid cylinder external of said conduitsurrounding said rod and receiving said piston, said piston subdividingsaid cylinder into a flushing compartment between said piston and saidseal and an actuating compartment on the other side of said piston; areservoir containing a flushing fluid; and a flushing duct connectingsaid reservoir to said flushing compartment for drawing said fluid intosaid flushing compartment upon displacement of said element in onedirection and for expelling said fluid from said flushing compartment ondisplacement of said element in the other direction.
 13. The slide valvedefined in claim 12 wherein said valve element is provided with arespective rod on opposite sides thereof and extending in the directionof displacement of said valve element, each of said rods being providedwith a respective piston, seal, fluid cylinder and flushing duct asdefined.